1. Technical Field of the Invention
The present invention relates to a corona discharge device which is utilized for charging the surface of a photoconductor of an electrostatic image forming apparatus to a desired potential.
2. Description of Related Art
A corona discharge device has heretofore been utilized as a charging device for charging the surface of a photoconductor of an image forming apparatus such as copying machines and printers to a desired potential.
There are a variety of corona discharge devices such as a wire discharge type device using a wire electrode, and a pin-array discharge type device wherein a plurality of projecting electrodes such as needle-shaped electrodes or sawtooth electrodes are used. In the case of the pin-array discharge system, the amount of ozone generation is smaller compared with the wire discharge system. In recent years, the pin-array discharge type of device is, therefore, being used widely, and a variety of this type of devices have been proposed.
However, there is a problem in the pin-array discharge system that a photoconductor is not charged uniformly in a direction of line since discharge electrodes are spaced at a predetermined distance in an axial direction of the photoconductor and a discharge is carried out concentratively from the tip end of the projecting electrode. An irregular charge causes blurring of an image in an image forming operation and lowers the quality of the image.
In order to solve such a problem of irregularity in charging, it has been devised to provide the projecting electrodes farther away from the surface of the photoconductor, or to make a distance between each of the projecting electrodes smaller by increasing the number of electrodes. However, there still remains a problem that a charging effect is lowered in the former, and in the case of the latter, the amount of substance generated by discharge is increased. Thus, the advantage of the pin-array discharge device is offset eventually.
For the same object, a scorotron discharge system is adopted to perform a corona discharge toward a member to be charged from projecting electrodes through a grid electrode wherein the grid electrode controlled under a constant voltage is arranged between the surface of a photoconductor and the portion where tips of the projecting electrodes are aligned. However, there still remains a problem that in order to obtain uniformity in charging in a practical use, the total amount of current needs to be increased necessitating a further improvement. A problem is further observed that there occurs irregularity in a charging process since a foreign substance is adhered to the tip of the electrodes, and the tip is rounded off as it changes with the passage of time.